Add Pool option `initialSize` which create connections when the pool is started.

(Please excuse my poor English. It's my first pull request. :])

I have been working to improve the pool features.

1. Add Pool option `initialSize` which create connections when the pool is started.
2. .bind() change to closure function. (closure is 10x faster than bind)
3. Clear the remaining queries from connection(queue) before release the connection.
4. Fix some with refactoring.